302 Computer Architecture and Organization
In Chapter 5, during the discussions of processor basics, all these effects were discussed but not the
reasons behind those. In Chapter 9, we have discussed about micro-programming and micro-architecture,
which might give some insight to the reader about the reasons behind these activities. In this chapter, we
shall discuss in details how all these functions are implemented and co-ordinated by a processor. As a mat-
ter of fact, the control unit (CU) within any processor is primarily responsible for initiating and maintain-
ing all these activities. In this chapter, we shall study some of the basic functions of this CU.
At this stage, the reader should note that there is no standardization of the architecture and organization
of any CU. As a matter of fact, each processor has its own unique design-approach and design-standard
for its CU. Moreover, in many cases, this CU design is governed by one important factor, compatibility
with previous processors of the manufacturer. For example, Pentium 4 is an evolution from Intel 8086 and
compatible with it. The program code developed for 8086 would be executed by Pentium 4 without any
trouble. This compatibility factor often places a restraint to the system designers. However, some of the
major details of any processor may be taken as universal. For example, every processor fetches, decodes
and executes instruction, one after another. The individual names (of different registers or busses) may be
different in various cases but the underlying principles and concepts remain unchanged. Therefore, in this
chapter we shall adopt a general approach for discussing the functions of CU and try to correlate those with
some established processors, as and when necessary as well as possible.
10.2 CONTROL UNIT (CU)
It is already known to us that any processor has several internal components for its functioning. Some
of these well established internal components (also referred as architectural blocks) are
R The arithmetic-logic unit or ALU
R Set of registers (some are general-purpose and a few are for speci c purpose)
R Internal bus
R Interrupt handler
R Clock and oscillator circuit
R Instruction decoder and
R The control unit or CU).
We have already discussed about the duties of these modules in the previous chapters. For example,
the ALU takes care of all arithmetic and logical operations as directed through its control signals gener-
ated by the CU. The set of registers accept or send out data as per the commands of the control signals
from CU attached with individual registers. Interrupt handling module reacts against eventual interrupts
as per the direction of control signals received from CU. Instructions are fetched, decoded and then
executed according to the status of control signals initiated by CU. Therefore, it may be concluded that
the CU is responsible of the smooth functioning of all other architectural modules of any processor.
In Chapter 9, we have brie y introduced this CU and its functioning related only to some instruction
execution. However, in that chapter, through Figure 9.4, we have introduced the duty-cycle of a processor.
Figure 9.4 was designated as a simpli ed duty cycle, as in reality, the duty cycle of any processor incor-
porates many more details and complications. However, there is no doubt about it that fetching, decoding
and executing instructions from primary memory are the major duties of the CU. Categorically, various
functions of a processor may be divided into following few groups:
R Fetch, decode and execute instructions.
R Interact with external memory.
R Interact with external I/O.
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